The field of the invention is silica gel esterification catalysts.
The state of the art of preparing esters of carboxylic acids in the presence of silica gel catalysts may be ascertained by reference to U.S. Pat. Nos. 3,364,251 and 3,617,226, and British Pat. No. 1,053,164. The state of the art of preparing silica gel catalysts may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", 2nd Ed., Vol. 18 (1969), pages 61-72, under the section "Silica" (Amorphous), and by reference to U.S. Pat. Nos. 2,384,946 of Milton M. Marisic, 2,900,349 of Albert B. Schwartz, and 3,642,659 of Ludwig Dorn et al, French Pat. No. 2,010,775, published Feb. 20, 1970, and West German Application No. 1,187,588 of Gerhard Heinze et al, published Feb. 25, 1965 and 2,100,220 of Ernst Podschus, published July 27, 1972.
U.S. Pat. No. 2,384,946 discloses the preparation of a body of catalyst pellets comprising hard homogeneous porous dried gel particles bounded by smooth hard glossy surfaces consisting substantially of smooth curves and characterized by a high resistance to attrition loss, said particles having been produced by forming a hydrosol or inorganic oxide characterized by an inherent capacity to set to a hydrogel upon the lapse of a suitable period of time without addition to or subtraction from said sol of any substance, admitting said sol in the form of separate globules to a body of a fluid medium substantially immiscible with water in which said globules assume spheroidal shape due to surface tension at the interface between said sol and said fluid medium, said medium being naintained at a temperature below the boiling point of said sol, retaining said spheroidal globules in said medium until gelation occurs, retaining in said globules substantially all the constituents of said sol until gelation occurs, washing the spheroidal hydrogel and drying the washed hydrogel.
According to U.S. Pat. No. 3,642,659 porous, abrasion-resistant bead-like catalyst supports containing in a matrix of a silicon dioxide gel mixed with 0.1 to 3 percent by weight of hydrated magnesium oxide (a) silicon dioxide filler with a specific surface area of 20 to 200 m.sup.2 /g in quantities of from 20 to 60 percent by weight and (b) an argillaceous mineral selected from the group consisting of kaolinite, montmorillonite and attapulgite in quantities of from 5 to 30 percent by weight, both quantities based on total contained solids are produced by suspending a solid in an aqueous stable silicon dioxide sol with a specific surface area of 150 to 400 m.sup.2 /g, which solid comprises (a) a silicon dioxide filler with a specific surface area of from 20 to 200 m.sup.2 /g in quantities of from 20 to 60 percent by weight and (b) an argillaceous mineral selected from the group consisting of kaolinite, montmorillonite and attapulgite in quantities of from 5 to 30 percent by weight, based on the total solids in the sol; mixing the resulting suspension with hydrated magnesium oxide in quantities of from 0.1 to 3 percent by weight based on the total solids in the sol; dividing the resulting gelable mixture in droplet form in a water-immiscible liquid to effect gelation of the droplets; separating the dry solid material from the liquid and drying and heating the resulting bead-like granulated material for at least ten minutes at temperatures of from 500.degree. to 1000.degree. C.
An object of the present invention is active, highly abrasion-free and mechanically stable silica gel esterification catalysts.
Esterification catalysts based on silicic acid are well known. Especially effective is the esterification of terephthalic acid with methanol in the presence of such esterification catalysts, since terephthalic acid has an unusually high melting point, but is chemically only slightly active and moreover is very difficult to dissolve in the usual solvents. These usual solvents include the low aliphatic alcohols. Accordingly, the esterification of terephthalic acid is more complex than that of the other benzenedicarboxylic acids, and, therefore, a special need exists for a suitable esterification method.
It is e.g., also very difficult to esterify trimethyl adipic acid, since here we have a combination of a difficult reaction because of a structural limitation, and a thermal instability. Over 200.degree. C, a rapidly increasing decarboxylation sets in.
Thus, West German Pat. No. 1,090,641 of Wilton H. Lind discloses the esterification of gaseous terephthalic acid with gaseous methanol in the presence of pulverulent silicic acid at a temperature of approximately 300.degree. C. West German Pat. No. 1,188,580 and the corresponding British Pat. No. 1,053,164 disclose also the esterification of gaseous terephthalic acid with gaseous methanol in a bed of solids filled with silicic acid in particle form, whereby the favorable method of charging the stream of methanol vapor with gaseous terephthalic acid permits a largely arbitrary standard of the molar ratio of terephthalic acid:alcohol. West German Pat. No. 1,083,474 corresponding to U.S. Pat. No. 3,364,251 teaches the reaction of pulverulent terephthalic acid with gaseous methanol in a fluidized bed of solids made up of pulverulent esterification catalysts such as silicates, hydroxides, oxides or phosphates. The fluidized bed is swirled up by a carrier gas. According to West German Pat. No. 1,224,313 corresponding to U.S. Pat. No. 3,617,226, the esterification of pulverulent terephthalic acid with gaseous methanol in the presence of silica gel in particle form succeeds in a rotary furnace whereby this esterification method, unlike the fluidized-bed process, is not bound or limited by particular gas velocities. West German Application No. 1,933,946 published Jan. 21, 1971 and corresponding to U.S. Application Ser. No. 39,761, filed May 22, 1970, inserts a rotary tube provided with a blade construction and heating surfaces into the fluidized oven bed. Herein, pulverized terephthalic acid with gaseous methanol is finely dispersed by a blowing action, and this mixture is immediately led through a bed of solids with an esterification catalyst in particle form, whereby any desired throughput can be managed, i.e., the speed and the staying times are optionally variable.
This esterification method with solid esterification catalysts, whereby the use of particulate silicic acid proves to be very advantageous, overcomes the main difficulties of the usual terephthalic acid esterification, as e.g., discontinuous operation methods, corrosion through acid catalysts, extremely long reaction times alcohol decomposition through formation of ether and olefines and expenditure of high pressure apparatus. Rather, the esterification in the presence of silicic acid can be accomplished continuously and without pressure, whereby the reaction expires in a few seconds. The silicic acid method works with very temperature reactions, and since the average staying time of the reaction products amounts to only a few seconds, no thermal decomposition occurs. Thereby, the end product which results from the synthesis with practically theoretical output is very pure.
Now, it is a general principle applicable to this SiO.sub.2 process that the catalyzation output is a direct function of the active surface of the silicic acid contact and that the size and distribution of pores is a great depending factor. This means that the particularly active silica gel varieties used are those that have the largest inner surface formed by the capillary walls, and these are the varieties that are commercially described as fine-pored.
As a disadvantage in the above-mentioned esterification process which takes place in the presence of silicic acid, the decomposition or the abrasion of particulate silicic acid is seen. The small particles of silica gel undergo abrasion which, in the rotary furnace, in the fluidized bed and in the bed of solids, are moved either by the gas stream or also mechanically. In addition that, the silica gel particles disintegrate under the esterification conditions, i.e., especially at high temperature and when water is present. This reaction is particularly observed when the fine-pored silica gel varieties are used which have a large inner surface and are, therefore, particularly active. The result is that, with a continuous process, fine contact dust settles in the contact bed and this leads to channel formation and, therefore, results in a no longer homogeneous distribution of the gas stream. Simultaneously, after a longer continuous time, an increase in gas resistance occurs in the contact bed and in the after-reactor system.
The above-mentioned limitations which are caused by particle decomposition, particle dust respectively of the silica gel contact in particle form, can largely be overcome by replacing fragmented silica gels with manufactured materials such as cylindrical materials which are made according to an arbitrary formation process of pure silicated brine after dehydration and, if necessary, calcination, are preserved in solid, dry active form (e.g., see Ullman, "Encylcopadie der technischen Chemie", Vol. 15, pp. 723,724 (1964), and are subsequently subjected to an aftertreatment for the hardening of the surface. This aftertreatment, as described in West German Pat. No. 1,667,430 and corresponding French Pat. No. 1,585,305 consists of periodically treating the gel particles with alcohol vapors which can contain small portions of water or acetic acid, when the temperature is increased.
It is also known that the abrasion resistance of the manufactured silica gels can really be increased by dispersing finely divided solid admixed materials in the silicic acid-hydrosol used in the production of the gel (West German Pat. No. 1,096,336), whereby admixed materials with an average particle diameter of between 1 and 5 .mu. are used. Suitable as such admixed materials are oxide gels, but also sand, soot, clay, graphite, metals, oxides silicates, phosphates, fluorides, sulfides, carbides, and several inorganic compounds. Hereby, however, there is no limitation to only very definite particle sizes of the admixed materials. but it is generally a matter of materials that are chemically different from the hydrosol and, therefore, in catalytic processes, often yield undesirable reaction characteristics. But above all, the inner surfaces of these gels, which decisively affect the activity of the catalyst, are greatly reduced. The esterification activity of these very abrasion-proof silica gels is therefore, very slight. When this deficiency is compensated for by increasing the temperatures over 330.degree. C, then we see more and more side reactions, especially a methanol splitting with formation of dimethylether which, at temperatures over 340.degree. C, further disintegrates with formation of formic aldehyde and methane. Finally, under esterification conditions and with increased temperature these by-products form carbon, whereby the durability and the activity of the catalyst are decisively and disadvantageously affected.
Also, West German Pat. No. 1,767,754 corresponding to French Pat. No. 2,010,775, published Feb. 20, 1970 and U.S. Pat. No. 3,642,659 discloses the formation of such spherical silica gels which are distinguished by an extremely high mechanical strength, i.e., breaking strength and abrasion resistance, and additionally by a high thermal stability. These pearl granules are produced according to the well known sol/gel process; by suspending solid substances in an aqueous, stable silicon dioxide sol with a specific surface of from 150 to 400 m.sup.2 /g according to BET; by mixing the suspension obtained with an aqueous paste of hydrated magnesium oxide in quantities of from 0.1 to 3 percent by weight of MgO, relative to the water-free granule; by dividing this gellable mixture into drops of the required size; by gelling these drops in a water immiscible liquid; by separating the granule from the liquid; by drying and calcining, whereby suspended in the silicon dioxide sol are a silicon dioxide containing filler with a specific surface of from 20 to 200 m.sup.2 /g according to BET in quantities of from 20 to 60 percent by weight relative to the dry granule, and argillaceous material from the group of kaolinites, montmorillonites and attapulgites in quantities of from 5 to 30 percent by weight. The suspension acquired is gelled to a bead-like granular material by adding hydrated finely divided magnesium oxide and the dispersion of the suspension in droplet form in water-immiscible medium, and the granule is subsequently dried and hardened from at least 10 minutes at a temperature of 500.degree. C to 1000.degree. C. Therefore, amorphous silica- or silicate fillers may be used as pore-filling frame substances whereby the supporting effect of the fillers on the gel is enhanced by relatively little addition of argillaceous materials.